Submitted to: Journal of Applied Clay Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2000
Publication Date: N/A
Interpretive Summary: Herbicides that are highly soluble in water, minimally sorbed by soil particles, and not readily degradable can move rapidly with water and hence, have a high potential to be found in surface and ground waters. Many acidic herbicides, such as dicamba, have these characteristics, and thus if accidentally reach high concentrations in soil as in an accidental spill, they have to be immobilized as soon as possible to avoid further potential water contamination. The potential off-site movement of these agrochemicals can be decreased by creation of sorptive or immobilizing zones in the soils by incorporating an appropriate adsorbent in the affected area. Synthetic organic clays (OCls), obtained by exchange of the original inorganic exchangeable cation on the natural clay by large alkylammonium ones, showed an increased adsorption capacity for polar pesticides in water. The objectives of this work were to assess the adsorption capacity of different OCls for dicamba and to test their abilit to immobilize it in soil. We found that by increasing the size of the organic group on the clay, the amount of dicamba sorbed on the clay increased. The treatment of an artificially dicamba-contaminated soil with highly adsorptive OCls rendered a dramatic decrease in the mobile dicamba, suggesting those OCls as potential immobilizing agents. These are promising initial results for scientists who will use them to develop cost-effective OCls, which can be used for remediation of contaminated sites.
Technical Abstract: The adsorption-desorption of the herbicide dicamba by organoclays (OCls) was studied at different concentrations and pH levels. Two smectites (SAz and SWy), varying in surface properties, were reacted with amounts of different alkylammonium cations [octadecyl (C18)-, hexadecyl-trimethyl(HDT)- and dioctadecyldimethyl(DOD)ammonium] equal to 50 0or 100% of the clays' CEC. Some sorption isotherms of dicamba on the OCls were of L-type and others of S-type, but both resulted in sigmoid form when extended to higher concentrations. Adsorption was greater for OCls with high layer charge, basal spacing, alkylammonium size and organocation saturation close to CEC. Dicamba adsorption by OCls seems to involve hydrophobic and polar interactions for which the availability of interlayer room between organocations is very important. Adsorption data at different pH levels and two different concentrations (0.05 and 1 mM) indicated that molecular dicamba is the main adsorbing species, especially at high concentration. Desorption isotherms were reversible, except those OCls with a primary (C18) and the largest quaternary (DOD) alkylammonium cations, for which there were moderate hysteresis effects as a result of stronger polar contributions in the primary alkylammonium and the difficulty for diffusion out in the case of the bulky quaternary cation. The treatment of an artificially dicamba-contaminated soil with highly adsorptive OCls rendered a dramatic decrease in the CaCl2-released or mobile dicamba, suggesting those OCls as potential immobilising agents. The amount of herbicide immobilised by the OCl was partially extractable with methanol/CaCl2 solution, suggesting its bioavailability and hence its possible combination with bioremediation techniques.